As shown in FIG. 4, a conventional solar cell device is provided with a semiconductor board 21, a diffusion layer 22, an antireflection layer 23, a back electrode 24, and a surface electrode 25.
For example, the solar cell device is produced as described below. The diffusion layer 22 comprising impurity, and the insulating antireflection layer 23 consisting of silicon nitride, silicon oxide, titanium oxide and the like are layered upon a light-absorbing side (surface side) of the semiconductor board 21 consisting of silicon in order. The semiconductor board 21 contains semiconductor impurity such as boron of 1×1016-1018 atoms/cm3 and presents one electro-conductive type (e.g., p-type) with specific resistance of about 1.5 Ωcm. Single crystal silicon is produced by a pulling up method and poly crystal silicon is produced by a casting method. Poly crystal silicon can be mass-produced and poly crystal silicon is more advantageous than single crystal silicon in production cost. The semiconductor board 21 can be obtained by slicing a piece with a thickness of about 100 to 300 μm of ingot produced by the pulling up method or the casting method.
The diffusion layer 22 is formed by diffusing impurity such as phosphorus on the light-absorbing side of the semiconductor board 21 and presents contrary electro-conductive type (e.g., n-type) to the semiconductor board 21. The diffusion layer 22 is, for example, formed by placing the semiconductor board 21 in the inside of a furnace and heating the semiconductor board 21 in a gas containing phosphorus oxychloride (POCl3).
The antireflection layer 23 is formed on the light-absorbing side of the diffusion layer 22 in order to possess the antireflection function and protect the solar cell device. The antireflection layer 23, which is consisted of silicon nitride membrane, for example, is formed by plasma enhanced chemical vapor deposition in which a mixing gas of silane (SiH4) and ammonia (NH3) becomes plasma by the glow-discharge and the resultant matter is accumulated. For example, taking consideration of a difference of refractive index from the semiconductor board 21, the refractive index of the antireflection layer 23 is made to a range of about 1.8 to 2.3 and the thickness of the antireflection layer 23 is made to a range of about 0.05 μm to 1.0 μm.
The surface electrode 25 is formed on the surface of the semiconductor board 21 and the back electrode 24 is formed on the back of the semiconductor board 21. The surface electrode 25 is formed by printing, drying and sintering an electro-conductive paste which contains an electro-conductive particle, an organic binder, a solvent, a glass frit and an optional matter. The back electrode 24 is also formed by printing, drying and sintering the electro-conductive paste. It is not necessary for the back electrode 24 to use the same electro-conductive paste as the surface electrode 25. Particularly, the surface electrode 25 plays a part as fire-through and it is important for improving the functions of the solar cell to adopt an appropriate composition and an appropriate sintering condition of the surface electrode 25. This fire-through is a following phenomenon:
In sintering, the glass frit contained in the electro-conductive paste works for the antireflection layer 23 and the antireflection layer 23 is dissolved. As a result, the surface electrode 25 and the diffusion layer 22 touch each other and an ohmic contact is obtained between the surface electrode 25 and the diffusion layer 22.
If the stable ohmic contact is not obtained between the surface electrode 25 and the diffusion layer 22, an electric resistance in series increases and a curved factor (FF) tends to decrease. Since the transformation efficiency of the solar cell is obtained by multiplying open-voltage by short-circuit electric current density and FF, if FF becomes smaller, the transformation efficiency of the solar cell is decreased.
Well, the properties of the cell are important for enhancing the power characteristic of the solar cell. For example, the power efficiency is increased by decreasing the electric resistance of the electrode. In order to attain the object, the following electro-conductive paste is disclosed in patent publication 1:
The electro-conductive paste contains an organic binder, a solvent, a glass frit, an electro-conductive particle, at least one metal or a metallic compound selected from a group of Ti, Bi, Zn, Y, In and Mo. The average particle diameter of the metal or the metallic compound is above 0.001 μm and under 0.1 μm.
Patent publication 1 states that it is possible to form a surface electrode having a stable high electro-conductivity and a good adhesion between the semiconductor and the electro-conductive paste by intervention of the antireflection layer, as a result of sintering of the electro-conductive paste containing the super-minute metal particle or the metallic compound. But, depending on the composition of the electro-conductive paste, as particularly shown in patent publication 1, after printing and drying the electro-conductive paste containing the super-minute particle metal or the metallic compound on the surface of the semiconductor board, the coating film (the paste film) shrinks and the contact resistance is increased by sintering. In one case, micro crack may be generated the surface of the semiconductor board by difference of thermal shrinkage action (coefficient of linear expansion) between the paste film and the semiconductor board. If the contact resistance is increased, as described above, FF becomes smaller and the transformation efficiency of the solar cell is decreased.